The present invention relates to the use of intersecting subresolution features in the creation of an image on an image plane, and more specifically to the use of intersecting subresolution features (and the light transfer properties associated therewith) on photomasks used to create micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer.
The process for transferring an image from a photomask to a silicon substrate or wafer is commonly referred to as lithography or microlithography. A photomask is comprised of a substrate (e.g., quartz) and an attenuator (e.g., chromium), with the pattern of the attenuator material being representative of the image desired to be formed on the image plane (e.g., a layer of photoresist material on a silicon wafer). The photomask is placed between an optical system and an energy source. The energy from the energy source is inhibited from passing through the areas of the photomask in which the attenuator material is present. The optical system projects a scaled image of the pattern of the attenuator material onto the image plane. The solubility of the photoresist material is changed in areas exposed to the energy. In the case of a positive photolithographic process, the exposed photoresist becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photoresist becomes insoluble and unexposed soluble photoresist is removed.
After the soluble material is removed, the resist image is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product.
However, as the resolution limits of the optical systems are approached, the actual transferred image becomes less and less a reduced representation of the photomask attenuator pattern. Several approaches to correct this optical distortion have been proposed and are collectively referred to as optical proximity correction or OPC. Optical proximity correction techniques modify the pattern of attenuator material on the photomask in such a way as to achieve the desired image at the image plane. Examples of well known optical correction techniques include serifs (described in U.S. Pat. No. 5,663,893 to Wampler) edge biasing, and edge jogs.
One embodiment of U.S. Pat. No. 5,362,584 to Brock et al. illustrated in FIG. 5 therein, is directed to the use of a plurality of phase-shifting regions 12e arranged in a grid-like arrangement. Phase-shifting regions 12e have circumferential edges spaced apart by a minimum distance d, with the resulting image formed on substrate 15e being a substantially rectangular image 16e. Similar embodiments are shown in FIGS. 6 and 7.
U.S. Pat. No. 5,698,349 to Yang et al. is directed to the fabrication of a phase shift mask comprised of a quartz substrate, a layer of chromium, a layer of phase shift material (e.g., silicon dioxide), and opaque spacers of sputtered chromium. The phase shift material overhangs the edges of the chromium by 500-5000 Angstroms. Light passing through the photomask is focused on the surface of a silicon wafer, with the alleged invention providing improved image resolution. However, there is no disclosure in the Yang Patent directed to the use of intersecting subresolution features to form desired images on an image plane.
In short, nowhere in the prior art known to the applicants have intersecting subresolution features and the light transfer properties associated therewith been used to create images on an image plane.